In the kitchen, oftentimes we desire to thicken a sauce or a broth without significantly changing its flavor. There are several ways to do this, and the physicochemical principles behind them are quite different in many cases. One way of thickening things is just to reduce them (i.e., boil them down), but that often involves chemical changes that alter flavor.
Other ways of thickening things including adding small amounts of rather bland ingredients that cause the sauce or other material to become thicker without extreme heating, or to create a complex emulsion that thickens materials due to physical rather than chemical changes. We shall examine some of both this evening.
First, we should investigate the interstellar terrorist threat. This is ONLY satire, but I can not pass up a good joke when one occurs to me, and this one involves quite a nice sight gag. I have reason to believe that the foiled Portland, OR planned attack was not actually planned by a religious extremist, but actually by a Kazon operative posing as one. For those of you unfamiliar with the Star Trek: Voyager series, the Kazon are inhabitants of the Delta Quadrant and are quite warlike, sort of like Klingons without much honor.
The evidence that I present is a photograph of the accused, would be terrorist first, and a photograph of an actual Kazon from the Delta Quadrant. Obviously, there has been a bit of surgical alteration to make the would be terrorist appear to be human, but it obvious that he is actually a Kazon. I changed my mind and did not post the pictures for fear of being thought of being in poor taste about a serious matter. If anyone is interested, I can include them as a reply to a comment.
The questions are: how did he cross 70,000 light years to get here, and why? My guesses are that there was some cooperation with the Borg to cross such vast distances as to the how, and to interfere with human scientific and technological advancements as the why. In any event, we have more to worry about than just human enemies.
Well, enough at this poor attempt at humor. We have food to thicken! As mentioned earlier, some things can be thickened just by reducing them, but obviously there has to be some sort of nonvolatile material to concentrate for this to happen. One simple example is taking sugar water and boiling it until it becomes a syrup. With careful temperature control the flavor is not much changed, but this is hardly an acceptable method to use to thicken a gravy or non sweet sauce, since that much sugar would certainly change its flavor significantly.
One common was to thicken savory (and, for that matter, sweet) sauces (I use the term henceforth to include gravies) is to add some kind of starch to it and cook it a little, until the starch gelates. Gelation is the process wherein starch (and some other food molecules) begin to interact with water in such a manner that the starch becomes very hydrated and the long starch molecules (high polymers of glucose, a simple sugar) begin to unwind from a highly organized structure to a disorganized one where these long molecules, with their attached water molecules, begin to entangle with each other, forming a three dimensional network that entraps more water droplets (remember, most food is predominately water) and thickens.
Starch can be added in several forms. Two of the most common in use are wheat flour and cornstarch. For proper flavor, wheat flour needs to be precooked before adding to the sauce, or the sauce will taste like raw flour unless it is cooked for a very long time, and the risk of sticking becomes quite real then. Usually flour is made into a roux first, then the liquid is added. A roux is a mixture of flour and some kind of fat that is cooked until the flour no longer has a raw flavor. Roux can range in doneness from white (just cooked long enough to dispel the raw flour taste) to almost black (one of the secrets to Cajun cooking). Now, it is not quite that simple to make a good roux.
For example, a classic white sauce is made with butter (novice and poor cooks sometimes use margarine), flour, and salt, along with other seasonings better added later to keep them from scorching. I have made roux for many, many years and find that salt is necessary during cooking the flour in the fat to make a good one. I believe that is due to the fact that salt inhibits the formation of gluten from the proteins in the flour, assuring a lump free final product. I also think that the flavor is improved when salt is added during the initial step as well.
To make a good roux from butter, it is necessary to cook all of the water out of the butter first. Butter in the United States is about 88% milkfat and 12% water, with a little bit of milk solids (sugars and proteins) as well. If that water is not removed before the flour is added, it will gelate the starch in the flour before the flour finishes cooking, and with only 12% water, local concentrations will build up and practically guarantee a lumpy final product. So, either use clarified butter or heat regular butter up until all of the water boils away, but do not brown it.
Then add flour and a little salt and heat gently with constant stirring (in a heavy skillet, I prefer cast iron) until the raw flour taste is gone but the flour has not browned at all. Then with constant, vigorous stirring, add milk and cook the mixture until it comes to the simmer. White pepper (if you do not like the black flecks from regular pepper) can then be added, and other seasonings to suit your particular project. I prefer to stir with a fork, but a whisk is also good. In my experience the trick is to put all of the milk in at once while stirring vigorously, and keep stirring until it is done.
By cooking the roux longer, deeper colors and more complex flavors and scents are produced, mostly by the Maillard browning reactions where protein and carbohydrate react at high temperature to produce hundreds of delicious aromas. For example, when I fry chicken I use vegetable oil for the fat (either soy or canola, but canola is a healthier choice) and remove the cooked chicken after it is done, keeping it warm in a slow oven. Then I drain off all of the fat except what I want to make the roux, carefully keeping the delicious browned bits of breading from the chicken in the skillet.
Then I add flour and a little salt and slowly cook the roux until it is just past golden brown. It is important not to cook it over too hot a fire, because if you scorch it bitterness sets in quickly. Have your milk ready to add, because you do not want to stop stirring. Add all of the milk (I often use half milk and half water) at once, stirring all the while. Gravies like this are a bit more robust than white sauces, so you can bring it to the boil rather than the simmer without harm. Just keep stirring to prevent lumps (except for the browned bits from the chicken) and to keep the gravy from sticking and scorching on the bottom of the skillet. Voila la! No lumps and no uncooked flour taste.
The other major way of introducing starch into a sauce is by using cornstarch. Unlike flour, cornstarch does not need to be cooked before addition, since it does not have a raw flour flavor. I made giblet gravy the other day like this:
I also added the drippings from the pan in which I had cooked the turkey (after separating the fat from them). This was too salty, but full of flavor, so all I had to do was dilute the gravy base with water to get the salt level right. I brought it to the boil, then stirred in a thin paste of cornstarch and cold water. Once it returned to the boil, it had thickened. It was not quite thick enough to suit me, so I stirred in a little more starch paste until I got it where I wanted it. Done!
By the way, if you have more gravy than you can eat, take out what you want to freeze before you add the starch. Freezing damages the network and thawed gravies tend to go quite runny. Just take out however much you want to freeze before adding the starch and freeze it, then when you want it later thaw it and add starch then
It is critical to add the cornstarch as a thin paste in water. If you do not hydrate the starch first, the surface of the dry starch will gelate on contact with water, forming balls (lumps) that, no matter what you do, will not go away. The only way to save a gravy when this happens is to strain the lumps out and try again.
There are other ways to add starch to liquids to thicken them. One way is to stir in something like cream of wheat or oatmeal (two old tricks for thickening things like chili or pasta sauce) and let it cook for some time. Neither of those have a raw flour flavor, so it is not required that you cook them over high heat like raw flour.
Another starch commonly used to thicken things like fruit pie fillings is quick cooking tapioca. Since it is already gelated, you can just stir it into the pie filling without fear of lumping, pour the filling into the crust, and than bake.
There are several other starches, like arrowroot, potato, and rice flour that have some uses, but overall potato starch seems to do the best job. Some of the other ones tend to get stringy, and stringy gravy is not to my liking.
A completely different class of thickening agents are proteins. The two most common ones are gelatin and eggs. Everyone is familiar with the fruit flavored dessert gelatin products, but actually you make gelatin every time that you cook animal bones and cartilage. Remember the drippings from my turkey? After I had set the container of it in the refrigerator to solidify the fat so that I could skim it off, the aqueous portion was much stiffer even than the dessert gelatin products. The gelatin is formed by moist cooking of collagen, the connective tissue in animals, breaking it down from a tough, almost inedible material to the smooth material that we know as gelatin.
That is the main reason that tough cuts of meat are most successfully cooked by slow, moist methods, like braising (aka pot roasting). If you check a pot roast with a cooking thermometer, it will be done as soon as it reaches around 160 degrees F. However, it will be so tough that you can not chew it! Allowing it to continue to cook in moist heat will convert the tough collagen to softer gelatin, and almost as if by magic the roast becomes very tender. By the way, that long cooking is too much for the vegetables, so add them at around an hour before serving time.
Gelatin is very, very forgiving, liquefying and resolidifying over and over as the temperature is changed. Unless you char it, it is almost impossible to ruin anything thickened by gelatin. Starch thickened materials are also fairly robust, within the limits that I mentioned earlier.
Eggs, on the other hand, are extremely delicate and have to be cooked with care. However, they are an excellent thickening agent for many, many dishes. The archetypical one is just eggs themselves. As eggs are cooked (slowly is always best), they go from a mostly watery mass to successively thicker and thicker materials, the extreme of which is the hard cooked (NOT hard BOILED!!!) egg. Well, just cooking eggs by themselves is fine, but they are also useful for thickening other foods.
Eggs thicken by a different process than starch. I already described how starch thickens (gelatin uses a similar mechanism). Eggs thicken because of the denaturation (aka coagulation) of proteins. However, the final result is a network of, in this case, coagulated protein that holds water within the network. Unlike starch or gelatin, foods thickened with eggs can easily be ruined by getting them too hot.
When that happens, the delicate network of coagulated protein degenerates into tight lumps, squeezing the water out of it as it collapses. Unlike starch or gelatin thickened materials, once the egg proteins have overcoagulated, there is no recovery of them because the protein network has irreversibly become compact. This is why custards can be so difficult to cook properly.
There is another problem with eggs. At the concentration normally used to thicken things, the protein in eggs it just about at its limit to begin with, and care has to be taken. If undercooked, the network never firms up properly and the result is too thin. If overcooked, the network collapses and the result is basically water with lumps in it. Undercooking is better, but just right is best. It is possible to make thickened foods just by using eggs, but it is easier to use some other ingredients to hedge your bets.
An old trick is to add some starch, either as flour or in other forms to reinforce the network of egg protein. Thus, traditional pie fillings and puddings are made with flour or cornstarch plus eggs, and pumpkin pie is reinforced by the pectins in the pumpkin itself. (Pectin is a complex carbohydrate that makes jelly gel). Tomatoes also have a lot of pectins and make fair thickeners. But we are talking about eggs.
It is almost impossible to thicken a food with egg unless it contains either salt or acid, such as lemon juice. It turns out that the proteins in eggs have an overall net negative charge, and so repel each other. When salt or acid is added, the positive ions from these materials neutralize some of the negative charge on the egg proteins, allowing them to knit together as they are cooked. One of the reasons that lemon and chocolate pie fillings are so easy is that both contain acidic ingredients (most folks do not realize that cocoa, except for the so-called “Dutch process” kind, is quite acidic).
Likewise, quiche contains a good deal of salt which stabilizes the egg protein network. So while eggs are excellent thickening agents, they are much more difficult to handle than most of the others. It takes practice to get it right. If you are really interested in learning how to thicken with eggs, take a standard egg custard recipe and play around with it until you can get it to turn out every time. It might be helpful if you have a dog or cat to keep the failures from going completely to waste.
There is yet another method to thicken foods, and it is entirely physical, with no cooking involved. This has to do with forming an emulsion between two fluids (usually, but emulsions involving solids are not unknown), either both liquids for a gas and a liquid. An emulsion is a physical mixture wherein one material (the continuous phase) supports billions and billions of small droplets of the material dispersed in it (the divided phase).
One of the most well known examples of a liquid in liquid emulsion is mayonnaise. In it water is the continuous phase and oil is the divided phase. You say, but, Doc, oil and water do not mix, at least for long! You are correct. We have a trick to make it work: an emulsifying agent. Generally, emulsifying agents have both fat and water attracting “ends” of their molecules. Technically, the water attracting end is called hydrophilic and the fat attracting end is called lipophilic (from Greek words meaning water or oil, and philos, “love”). Alternately, they are called lipophobic and hydrophobic, from the Greek root phobos, “fear”.
In mayonnaise, the emulsifying agent is egg yolk, particularly a component of it called lecithin. Lecithin has the proper two ends, and acts to stabilize the emulsion by reducing the repulsion betwixt the water and the oil. Interestingly, mayonnaise contains both salt and acid, necessary not only for flavor but to keep those pesky negatively charged proteins mentioned earlier from causing mischief. Most commercial mayonnaise also contains a little mustard, the starch in which also stabilizes the emulsion a little. Mustard also has a lot of pectin related carbohydrates that also stabilize the emulsion.
Making mayonnaise is easy, and illustrates how emulsions are formed. Interestingly, in mayonnaise the continuous phase is water, but only comprises a minor part of the dressing. Around 90% of the emulsion is oil, and the 10% water is happy to keep it suspended if a couple of precautions are taken. Let us make a pint right now as a thought experiment.
The way that I do it is to use either a blender or a food processor. You can also use a regular mixer, or even a whisk if you have strong arms. The first thing to do is to get the continuous phase ready to accept the divided phase. We do this by taking a whole egg (some recipes call for only the yolk, and it is the working part, but I have not been able to see much difference), two tablespoons of either vinegar or lemon juice (I like one of each), half a teaspoon of salt, and a squeeze (around 1 or two teaspoons) of yellow mustard.
Put that in your blender and whir it until it looks pretty well mixed. Now we are going to add a pint of salad oil, either soy or canola (olive oil gives erratic results for reasons that I will go into if there is any comment asking about it). Running the blender at a medium speed, with the plug taken out of the top (use the top or you will have mayonnaise coated walls), add a tablespoon of oil and blend until homogeneous. Now add another tablespoon and blend. Keep on doing that until you notice it start to stiffen.
What we just did was to prepare the continuous phase and begin to disperse the divided phase into it. Once the divided phase starts to form, you can add the oil faster. The reason is that if you put a whole lot of oil in to begin with, you never break it into small enough droplets to stay suspended. Once you have enough tiny droplets, they bump into the additional oil and help break it up. It is like you just added millions of extra blades to your blender. You can add the oil now in a steady stream, the rest of the pint going in over a couple of minutes.
Actually, that one egg and two tablespoons of vinegar would emulsify probably a quart of oil, but you really want some flavor from the acid and salt in the final product, so this makes a better tasting result. If you are nervous about raw egg, you can buy pasteurized ones in some stores that eliminate the threat of egg borne disease. Store the mayonnaise in the refrigerator and use within two weeks maximum.
I do not make mayonnaise any more because being alone I can not eat a pint of it in two weeks, so I opt for the store kind that has some preservative in it (and is also pasteurized) so I can eat it all before it goes off. If you have a large family and you like mayonnaise, you can save money by making it yourself, IF you eat it all before it expires. You can also adjust the flavor balance to obtain a product that you prefer.
My final example of a food thickened by forming an emulsion is whipped cream. In this case, the continuous phase is water and the divided phase is air! The fat and proteins in the cream are the emulsifying agents. Sugar is added just for flavor, and cream will whip just as well without it.
To whip cream, try to find whipping cream that has not been ultrapasteurized. Normal pasteurization is a more gentle process that does not damage the fats and proteins in the cream as badly and ultrapasteurization does. Untrapasteurization involves much higher temperatures than normal pasteurization and preserves the emulsifying powers of the fat and protein much better. You can whip untrapasteurized cream, but it collapses faster than that made from cream more gently treated.
At my place, the shelf life of whipped cream is rarely in issue!
It is important for all of your materials to be cold before you try to whip the cream. I put the bowl and beaters that I plan to use in the freezer for half an hour or so, and the cream itself for about 20 minutes (you do not want ice to form in the cream, because that damages the structure of the emulsifiers). When you are ready to go, pour the cream into the bowl and start beating it at high speed. While it is possible to whip cream by hand with a whisk, it is extremely difficult because everything gets warm before you can whip enough air into it, unless you go outside on a really cold day.
Whip until peaks begin for form, then however stiff you want it. Just before it gets stiff enough to suit you, whip in the sugar to taste. I recommend using powdered sugar if you are going to serve the whipped cream right away, because granulated sugar tends to dissolve slowly at the cold temperatures and relatively low water content (whipping cream is around 35% fat, give or take, so only around 65% water) it may be gritty. If you are going to hold it for half an hour or more, it does not matter. I like to whip in a little vanilla, but that is just my taste. Keep it cold until you serve it.
Do not whip past firm peaks, because you are likely to make butter instead of whipped cream. As the milkfat warms up in the kitchen, it is possible for the softened fat to congeal into bits of butter, ruining the whipped cream. This is one reason to start with really cold ingredients.
Whipped cream does not form as stable an emulsion as mayonnaise. There are a couple of reasons for this. First, the difference in mass of air and water is around 800 times less than the difference of mass betwixt oil and water, so the air tries to rise and escape because of nothing more than gravity. Second, in mayonnaise there is so much oil in it that the water scarcely has a chance to evaporate. In whipped cream, water is 65% of the liquid, so, just like a soap bubble, as the surface of the bubble dries out and thins, it pops.
I think that this has covered the subject sufficiently for tonight. There are a number of different ways to thicken things, but they all involve forming either a network of long molecules like starch and cooked egg do, or by forming emulsions where the divided phase exerts force on the continuous phase. I forgot to mention that the physics behind emulsions becoming thick has at least partially to do with the fact that the smaller the fluid particle, the more rigid the sphere, due to surface tension. In emulsified items, the billions of extremely small particles of the divided phase act almost like ball bearings, and impede the flow of both the continuous phase and the divided phase.
Well, you have done it again! You have wasted many perfectly good einsteins of photons reading this rather thick piece. And even though John Bolton actually says that the current administration might have done something correctly when he reads me say it, I always learn much more than I could possibly hope to teach by writing this series. Please keep those comments, questions, corrections, and other feedback coming! Remember, no science or technology issue is off topic in the comments. I shall stay on tonight as comments justify, and shall return tomorrow evening after Keith for Review Time to answer any late comment.
Warmest regards,
Doc
Crossposted at Docudharma.com and at Dailykos.com
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cooking science?
Warmest regards,
Doc